Certain tangible objects when formed from such elements as iron, cobalt nickel, and the like respond to the effect of a magnetizing force and become magnetic. This is often desirable; for example, in the production of permanent magnets and in the operation of electromagnets. In certain instances, however, this quality is not desirable.
One instance where this magnetic quality is not desirable is when the object is used as a measurement reference. Where the object is a gear in an engine, the gear's teeth can be used as a reference for one or more magnetic proximity pickups or sensors in monitoring the gear's revolutions per minute (RPM). Such monitoring of the gear can be used to control engine RPM, and to stop the engine in the event of an over speed condition. However, a gear that exhibits residual magnetism (here an undesirable quality) will interfere with the normal operation of the magnetic pickups when the gear's teeth act to vary the magnetic flux density sensed by the pickups.
Where an object such as a gear can be easily removed and demagnetized or replaced, the interruptive effect of this form of gear failure can be relatively small. But where the gear is internal to a large engine, such as a gas turbine engine, the interruptive effect can be large.
Gas turbine engines are key to modern propulsion systems for aircraft, land vehicles, and sea vessels. Any nonoperating time obviously becomes costly. The downtime to physically remove the gear (if not the entire engine to have easier access to the gear), demagnetize or replace the gear and to reinstall it, (and reinstall the engine in certain instances) is significant; and therefor is costly.